This invention relates to silicone sealants, and more particularly to sealants which include certain cross-linked methylhydrosiloxanes.
The cross-linking of silicones to form higher molecular weight polymers has been used to prepare many useful silicone products. One example is the hydrolysis of reactive chloro, alkoxy, and amino, silanes to form various fluids, resins and elastomers through silanol condensation. Low molecular weight resinous silicones have been formulated with high viscosity silanol fluids to prepare pressure sensitive adhesives. These pressure sensitive adhesives have medical utility such as surgical dressings as well as various non-medical applications. The cross-linking reaction of vinyl enblocked silicone fluids of about one thousand centistoke viscosity with methyl/hydrogen silicones in the presence of a platinum catalyst provides "psuedo" interpenetrating network gels. These gels are used in applications ranging from breast implants to paper coatings. Condensation curing has been employed to prepare room temperature vulcanizing sealants and adhesives. The room temperature vulcanizing and curable silicones have been composed of cross-linkers with moisture sensitive groups on silicon, and are typically catalyzed by tin, zinc, titanium, iron, or carboxylate salt catalysts. High temperature vulcanizing cures involves a method in which a peroxide initiates cure of a silcon hydride and an olefin substituted silicone at elevated temperature. Such technology has found application in penile prosthesis, for example.
U.S. Pat. No. 4,746,750, issued May 24, 1988, relates to the hydrosilylation of allyl methacrylate with trimethylsilane in the presence of a rhodium catalyst to provide bis(trimethylsilyl)dimethylketene acetal. The mechanism of the reaction involves the intermediate formation of trimethylsilyl methacrylate which undergoes further hydrosilylation by trimethysilane. This was indicated to occur as a result of the addition of the silane to the olefin to provide an adduct which suffered facile beta elimination of propene. However, the trimethylsilyl methacrylate was not isolated. Thus, the mechanism according to the '750 patent can be illustrated as follows: ##STR1##
Evidence of the intermediacy of trimethylsilyl methacrylate has been obtained by the reaction of other allyl esters with various silanes, for example, the reaction of trimethylsilane with allyl acetate and allyl butyrate. The reaction has also been carried out with silanes such as phenyldimethylsilane and dimethylchlorosilane which produce equivalent results. For example, sym-tertramethyldisiloxane provides the corresponding di-ester disiloxane shown below. This synthetic method is the subject of prior copending U.S. patent application Ser. No. 351,639, filed May 15, 1989 now U.S. Pat. No. 4,919,242. In accordance with the copending application, the mechanism can be illustrated as follows: ##STR2## in which R is methyl or ethyl, and Me and Et are methyl and ethyl, respectively.
The mechanism is believed to involve the hydrosilylative addition of the silane either to the internal olefin or to the carbonyl followed by elimination of propene, however, the mechanism for silyl ester formation has not been completely delineated. These reactions do, however, clearly demonstrate a novel method of exchanging the allyl group of an allyl ester for a silyl group of a silicon hydride.
Since one of the major cure reactions used in sealants is acetoxy hydrolysis known as moisture curing, it has been unexpectedly discovered that the foregoing technology for converting a silicon hydride to a silicon ester could be applied in a new and novel manner for creating an "in situ" acetoxy cure. Thus, and in accordance with the concept of the present invention, the silicon hydrogens on a silicone are converted to acetoxy groups and exposed to air, and the material cures and cross-links through silanols to form new siloxane bonds.
Silicone sealants are old in the art and such sealants generally include a mixture of a silicone polymer, one or more fillers, a crosslinking component such as a reactive silane, and a catalyst. The silicone polymer has a siloxane backbone and includes pendant alkyl, alkoxy, or acetoxy groups. Such groups are hydrolyzed to silanol groups which form larger chains by condensation. The sealants may be applied by means of a caulking gun and are cured by exposure in moist air. The cured products are employed as flexible space fillers. The silicone sealants have low shrinkage characteristics and may be applied and used over a wide temperature range. Representative formulations of such silicone sealants are set forth in more detail and may be found by reference to U.S. Pat. No. 3,642,692, issued Feb. 15, 1972; U.S. Pat. No. 3,697,473, issued Oct. 10, 1972; U.S. Pat. No. 3,734,881, issued May 22, 1973; and U.S. Pat. No. 4,711,928, issued Dec. 8, 1987. While the present invention relates to a silicone sealant, the sealant of this invention differs from prior art sealants in that a new mechanism is provided, as noted previously in detail, for achieving a cure. Thus, in accordance with the present invention, a separate crosslinking component is not required as the combination of the methylhydrosiloxane and the allyl acetate provides the acetoxy group which cross-links the siloxane to higher molecular weight polymers.